From 9b1ff689d4585547dbc5279f3388602e4e01a2ce Mon Sep 17 00:00:00 2001
From: crazycs <crazycs520@gmail.com>
Date: Tue, 15 Jan 2019 12:46:28 +0800
Subject: [PATCH] sessionctx/variable: fix select variable return wrong result
 when variable is only global scope variable (#8968) (#8992)

---
 executor/set_test.go        | 20 ++++++++++++++++++++
 plan/expression_rewriter.go |  8 +++++++-
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/executor/set_test.go b/executor/set_test.go
index bf7981df61a66..980f01379e7c4 100644
--- a/executor/set_test.go
+++ b/executor/set_test.go
@@ -244,3 +244,23 @@ func (s *testSuite) TestSetCharset(c *C) {
 	// Issue 1523
 	tk.MustExec(`SET NAMES binary`)
 }
+
+func (s *testSuite) TestSelectGlobalVar(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+
+	tk.MustQuery("select @@global.max_connections;").Check(testkit.Rows("151"))
+	tk.MustQuery("select @@max_connections;").Check(testkit.Rows("151"))
+
+	tk.MustExec("set @@global.max_connections=100;")
+
+	tk.MustQuery("select @@global.max_connections;").Check(testkit.Rows("100"))
+	tk.MustQuery("select @@max_connections;").Check(testkit.Rows("100"))
+
+	tk.MustExec("set @@global.max_connections=151;")
+
+	// test for unknown variable.
+	_, err := tk.Exec("select @@invalid")
+	c.Assert(err.Error(), Equals, variable.UnknownSystemVar.GenByArgs("invalid").Error())
+	_, err = tk.Exec("select @@global.invalid")
+	c.Assert(err.Error(), Equals, variable.UnknownSystemVar.GenByArgs("invalid").Error())
+}
diff --git a/plan/expression_rewriter.go b/plan/expression_rewriter.go
index 44c11336de67c..ccace4903db08 100644
--- a/plan/expression_rewriter.go
+++ b/plan/expression_rewriter.go
@@ -803,7 +803,13 @@ func (er *expressionRewriter) rewriteVariable(v *ast.VariableExpr) {
 	}
 	var val string
 	var err error
-	if v.IsGlobal {
+	sysVar := variable.SysVars[name]
+	if sysVar == nil {
+		er.err = variable.UnknownSystemVar.GenByArgs(name)
+		return
+	}
+	// Variable is @@gobal.variable_name or variable is only global scope variable.
+	if v.IsGlobal || sysVar.Scope == variable.ScopeGlobal {
 		val, err = variable.GetGlobalSystemVar(sessionVars, name)
 	} else {
 		val, err = variable.GetSessionSystemVar(sessionVars, name)